In epidemiology, a case fatality risk (CFR) — or case fatality rate, case fatality ratio or just fatality rate — is the proportion of deaths within a designated population of "cases" (people with a medical condition), over the course of the disease. A CFR is conventionally expressed as a percentage and represents a measure of risk. CFRs are most often used for diseases with discrete, limited time courses, such as outbreaks of acute infections.

For example: Assume 9 deaths among 100 people in a community all diagnosed with the same disease. This means that among the 100 people formally diagnosed with the disease, 9 died and 91 recovered. The CFR, therefore, would be 9%. If some of the cases have not yet resolved (either died or recovered) at the time of analysis, this could lead to bias in estimating the CFR.

A mortality rate — often confused with a CFR — is a measure of the number of deaths (in general, or due to a specific cause) in a population, scaled to the size of that population, per unit of time. (For example, a rate of 50 deaths per 10,000 population in a year resulting from diabetes. The mortality rate, therefore, would be 50:10,000 or 5:1,000.)

Technically, CFRs are actually risks (or "incidence proportions") and take values between 0 and 1. They are not rates, incidence rates, or ratios (none of which are limited to the range 0-1). If one wants to be very precise, the term "case fatality rate" is incorrect, because the time from disease onset to death is not taken into account. Nevertheless, the term case fatality rate (and the abbreviation "CFR") is often used in the scientific literature.